The present invention relates to a new and improved method of, and apparatus for, protecting an object, and specifically protecting ground shorts in the rotor circuit of a generator of an electrical network, wherein the rotor circuit, during normal operation, is shifted towards ground by a predetermined voltage, usually referred to as an injection or test voltage or signal, and as the injection voltage there can be used an alternating-current voltage which is at the network frequency.
Such type methods are part of the state-of-the-art. To explain this more fully, reference at this point will be made to a prior art arrangement as shown in FIG. 1 wherein the injection voltage Uv is delivered to the electrical system by means of the injection signal-transformer 3. A fault relay F then responds when, in the event of a ground short E, a current flows by virtue of the voltage Uv through the line 1. The capacitor K prevents the flow of a direct-current in the event of a ground short E. As the injection voltage Uv there has heretofore been selected a voltage which is at the network frequency or also a voltage which is not at the network frequency. The phase of such voltage does not change as a function of time. The drawback of this prior art technique resides in that regardless of the frequency of the voltage Uv there nonetheless exists the possibility that there can prevail a current having the same frequency as the injection voltage Uv and which flows through the capitances of the rotor circuit. With modern day generators employing thyristor excitation this possibility is increasingly great due to the large number of harmonics which arise at the rotor. Such disturbance current can cause faulty response of the protective device.